diff --git a/mozilla/xpfe/appshell/src/nsXULWindow.cpp b/mozilla/xpfe/appshell/src/nsXULWindow.cpp
index ab459d00b40..a6be57a8908 100644
--- a/mozilla/xpfe/appshell/src/nsXULWindow.cpp
+++ b/mozilla/xpfe/appshell/src/nsXULWindow.cpp
@@ -297,6 +297,7 @@ NS_IMETHODIMP nsXULWindow::ShowModal()
 
    window->SetModal(PR_TRUE);
    mContinueModalLoop = PR_TRUE;
+   EnableParent(PR_FALSE);
 
    nsCOMPtr<nsIJSContextStack> stack(do_GetService("nsThreadJSContextStack"));
    nsresult rv = NS_OK;